In radiography the interior of objects is reproduced by means of penetrating radiation, which is high energy radiation also known as ionizing radiation belonging to the class of X-rays, gamma-rays and high-energy elementary particle radiation, e.g. beta-rays, electron beam or neutron radiation. For the conversion of penetrating radiation into visible light and/or ultraviolet radiation luminescent substances, called phosphors, are used.
In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
In practice the light emitted imagewise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
More recently as described e.g. in U.S. Pat. No. 3,859,527 an X-ray recording system has been developed wherein photostimulable storage phosphors are used having in addition to their immediate light emission (prompt emission) on X-ray irradiation the property to store temporarily a large part of the X-ray energy. Said energy is set free by photostimulation in the form of fluorescent light different in wavelength from the light used in the photostimulation. In said X-ray recording system the light emitted on photostimulation is detected photoelectronically and transformed into sequential electrical signals.
The basic constituents of such X-ray imaging system operating with a photostimulable storage phosphor are an imaging sensor containing said phosphor in particulate form normally in a plate or panel, which temporarily stores the X-ray energy pattern, a scanning laser beam for photostimulation, a photoelectronic light detector providing analogue signals that are converted subsequently into digital time-series signals, normally a digital image processor which manipulates the image digitally, a signal recorder, e.g. magnetic disk or tape, and an image recorder for modulated light exposure of a photographic film or an electronic signal display unit, e.g. cathode-ray tube.
The terminology "radiographic screen" as used herein refers to screens suitable for use in conventional screen-film combinations or for use in stimulated luminescence radiography.
From the preceding description of said two X-ray recording systems operating with radiographic screens in the form of a sheet, plate or panel it is clear that said screens serve only as intermediate imaging elements and do not form the final record. The final image is made or reproduced on a separate recording medium or display. Said radiographic screens can be used repeatedly. Before re-use of radiographic screens containing a photostimulable phosphor the residual energy pattern is erased by flooding with light. The expected life of the phosphor screen is limited mainly by mechanical damage such as scratches.
Common radiographic screens comprise in order : (1) a support (also called substrate), (2) a binder layer comprising phosphor particles, usually having a particle size in the range of 1 to 40 .mu.m, applied from an organic solvent or solvent mixture comprising the binder in dissolved and the phosphor particles in dispersed form, and (3) a protective coating to protect the phosphor-containing layer against moisture and abrasion during use. Further, a primer layer is sometimes provided between the phosphor containing layer and the substrate to closely bond said layer thereto.
A radiographic screen whether it is photostimulable or not is generally prepared as follows :
phosphor particles are mixed with a dissolved organic polymeric binder in a suitable mixing ratio to prepare a dispersion. Said dispersion is uniformly applied to a substrate by a known coating technique, e.g. doctor blade coating, roll coating, gravure coating or wire bar coating, and dried to form a luminescent phosphor-binder layer. The phosphor-to-binder weight ratio is usually in the range of 80:20 to 95:5.
The coating of said phosphor binder layer comprising a large amount of organic solvent poses a serious problem of solvent recovery because organic solvents may not enter the environment.
The protective layer will almost be indispensable where the screens have to be handled in combination with radiographic films or are subjected to roller transport.
In modern hospitals, where a large number of X-ray eposures are made on a daily basis, automatic film changer devices are used wherein each lightsensitive film is fed into a cassette in contact with a pair of X-ray conversion screens. The feed path of the film changes abruptly near the entrance of the cassette and on entering the cassette the rim of the film touches the screens with sufficient mechanical force to cause damage to the phosphor layer if it were not protected by an abrasion resistant topcoat.
In particular radiographic recording embodiments applied e.g. in radiography operating with photostimulable storage phosphor screens the radiographic screens are handled without protective cassette and transported in the various process stations between rollers whereby the screens are subjected to considerable friction forces which may result in abrasion.
These features make that practically binderless phosphor-containing screens have never been considered for use in embodiments of radiography wherein the screens are subjected to friction, e.g. by contact with film or transport means.
The failure of the abrasion resistance of the phosphor-containing screen may result in defects such as scratches, cracks and dust formation whereby correct medical diagnosis becomes impossible and screen defects in industrial radiography can be interpreted as material failures of the nondestructively tested object.
Especially in medical radiography there is a demand to reduce the dose of X-ray radiation received by the patient without affecting the quality of the radiograph for diagnostic purposes. A main feature in reducing the X-ray dose is to dispose of a radiographic screen with high X-ray stopping power and high conversion of X-ray energy into fluorescent light. The X-ray stopping power depends largely on the kind of phosphor but also on its packing density in the screen, and the image sharpness which is a primary feature in correct diagnosis depends largely on the phosphor layer structure.
Blurring from lateral diffusion of light in the phosphor-containing layer is decreased as (1) the phosphor layer is made thinner, (2) fluorescent light absorbing dyes or pigments are present therein or coated as anti-halation layer, and (3) the size of the phosphor particles is decreased to increase internal scattering which will result in increased internal light absorption particularly when screening dyes or pigments are present (ref. The Fundamentals of Radiography, 12th. ed. Health Sciences Markets Division--Eastman Kodak Company Rochester, New York 14650, p. 58-59).
However, the packing density of the phosphor particles cannot be increased unrestrained since the presence of a binder is a conditio sine qua non for obtaining a sufficient coherence between the phosphor particles and adherence to their support in view of a necessary resistance to abrasion in handling or machine transport of the screens.
Taking into account the preceding in order to obtain radiographic screens with an X-ray absorption power and fluorescent light emission power as high as possible combined with a sufficient mechanical strength the binder content has to be kept at a minimum but has to be still high enough to fulfil the requirement for sufficient mechanical strength.
Binder-free phosphor layers, e.g. applied by vapour-deposition, are in practice solely reserved in the production of cathode ray tubes and X-ray image intensifier tubes (ref. the periodical Medicamundi, 19, No. 1, (1974) p. 3-7, wherein the phosphor layers cannot suffer from damage by mechanical wear. Moreover, as can be learned from the last mentioned periodical not every phosphor is suited for vapour deposition, and the obtained speed is generally too low for medical purposes since the phosphor grain size obtained by that coating method is particularly small.
According to another technique luminescent material is deposited binder-free by electrophoretic deposition from a mainly organic liquid forming phosphor layers for use in displays and CRT tubes as described e.g. in U.S. Pat. Nos. 2,851,408, 3,681,222 and J. Electrochem. Soc., Vol. 136, No. 9, September 1989, p. 2724-2727.
Phosphor particles suited for use in the production of radiographic screens as mentioned already must have sufficient X-ray stopping power and contain therefor heavy weight atomic elements (atomic number Z&gt;50) whereby these particles have a particularly high density, and are not easily dispersed in organic liquids.
A further problem is to deposit these phosphor particles to their support with sufficient adherence thereto and sufficient coherence to allow further coating with a protective layer as referred hereinbefore without damaging the phosphor particle layer.